Not a solution, but my view is...

Since 10.7, Apple have been making it progressively harder to change the background for the OS X login window. We have assumed from this that Apple really don't want you changing it. Since 10.6 we've just left it at its default, which is pleasant enough!

We still set a login window message but nothing more than that.

I made a script launchdaemon pair that checks for the file to be in /Library/Caches and if its not copy it from my safe location. So, it gets put back after every reboot. I have not seen it go away between logins but rarely do that.

You need to chmod the new com.apple.admin.png to 555 - works in my testing.

EDIT: meant 544.

644 works for me. i have no idea why a picture would need to be executable :D

D'oh, not what I meant at all - office party last night something something foggy something. 544 it is.

Have you tried replacing the default image in core services? We set this up in a .pkg that replaces the original default desktop image than runs a postinstall script

!/bin/bash

This file needs to be removed so we can create a link to your new default desktop

rm /System/Library/CoreServies/DefaultDesktop.jpg

replace "desktop.jpg" with your image.

ln /Library/Desktop Pictures/desktop.jpg /System/Library/CoreServices/DefaultDesktop.jpg

This will cause the system to recreate the blurry login image with your new default desktop

rm /Library/Caches/com.apple.desktop.admin.png

exit 0

This knocks out the old default and the cached file apple uses for the login window. When you restart the file in /Library/Caches will be recreated with your new DefaultDesktop.jpg. This also makes the image we picked be the lock screen image for users, so no worries what ever the user has chosen for they're desktop over riding the cached file when they are on the lock screen.

might as well not get too used to doing this SIP will prevent you from doing such in the new OS according to the presentation they gave at WWDC last week.

running chflgs uchg after setting the wallpaper made it stick for me. not my hint found it online

http://jacobsalmela.com/customize-os-x-yosemite-login-screen/ at the bottom last comment by @jacobsalmela

LS

@LSinNY that command did in fact get rid of most of the overlay. Much better...but yeah, probably won't stick around for long given the 10.11 changes.

sudo chflags uchg /Library/Caches/com.apple.desktop.admin.png